It does not have a birth scene. Once you get the big worm scene triggerd when not wearing panties when sleeping, the pregnancy progress would go back to 0% and nothing more.
Each time you get egged by Warfroach, egg-bee, egg-fly, and the Slug, you would get the parasites progress increased.
It require to get egged 3 times by any of them in order for the parasite to change to the big worm scene.
The fullness also does not change, since its a fixed value based on the type you are pregnant from. So no matter how many eggs you have, it does not increase it seems.